1. Field of the Invention
The present invention relates to a connection structure for superconducting conductors each prepared by assembling a plurality of superconducting wires, and more particularly, it relates to a connection structure for superconducting conductors employed in the field of power transportation, high-energy transportation, medical care or the like effectively utilizing superconductors.
2. Description of the Background Art
In order to apply superconducting conductors to a coil or the like, for example, it is necessary to connect the conductors with each other. If superconducting conductors each prepared by assembling a plurality of superconducting wires with each other are directly connected with each other by solder or the like, however, connection resistance is disadvantageously increased.
On the other hand, various studies have been generally made as to a connection structure for superconducting wires having small connection resistance.
In relation to structures of connected portions of metal superconducting wires, for example, such studies include a method of directly connecting superconducting filaments with each other for reducing connection resistance, as described in xe2x80x9cSuperconductivity Cryogenic Engineering Handbookxe2x80x9d (by Ohmsha, issued on Nov. 30, 1993) pp. 913 to 914. According to this method, the connection resistance is extremely reduced and hence the superconducting conductors can be applied to a nuclear magnetic resonance imaging apparatus employed for medical care or the like, for example.
As a connection structure for oxide superconducting wires, on the other hand, superconductors of metal-coated superconducting wires are connected with each other as disclosed in Japanese Patent Laying-Open No. 4-269471 (1992), or superconducting filaments of superconducting multifilamentary wires are connected with each other as disclosed in Japanese Patent Laying-Open No. 5-234626 (1993). According to this method, connection resistance is suppressed to allow persistent current joining.
Further, xe2x80x9cApplied Superconductivityxe2x80x9d (by the Nikkan Kogyo Shimbun Ltd., the first edition issued on Jul. 15, 1986, pp. 111 to 112) discloses a method of directly connecting filaments with each other while removing matrices for reducing connection resistance if superconducting wires must be connected with each other in an intermediate stage of wiring in an MRI coil driven in a persistent current mode which must avoid magnetic field attenuation.
However, every such prior art disadvantageously requires sophisticated technology for the connection, although small resistance can be attained as described above.
In the method disclosed in Japanese Patent Laying-Open No. 5-234626 (1993), for example, coatings of the multifilamentary wires must be removed for exposing portions of the superconducting filaments to be connected with each other, while it is predicted that this operation is complicated and difficult due to extremely small diameters of the filaments. Particularly in multifilamentary superconducting wires each having a number of filaments embedded therein, it is extremely difficult to connect such filaments with each other. Further, it is particularly difficult to expose filaments of oxide superconductors, due to inferior flexibility of ceramics.
In order to solve the aforementioned problems, an object of the present invention is to provide a connection structure for superconducting conductors which can simply connect superconducting conductors with each other with small connection resistance and no requirement for sophisticated technology.
The connection structure for superconducting conductors according to the present invention is characterized in that each of first and second superconducting conductors to be connected with each other includes first and second superconducting wires, each of the first and second superconducting wires includes a filament assembly containing superconducting filaments, the first superconducting wire of the first superconducting conductor is joined with that of the second superconducting conductor, and the second superconducting wire of the first superconducting conductor is joined with that of the second superconducting conductor.
Namely, the present invention mainly relates to a connection structure for connecting two superconducting conductors, each prepared by assembling a plurality of multifilamentary superconducting wires, with each other on end portions thereof.
While first and second superconducting conductors having identical sectional structures are connected with each other in general, the present invention is adapted to join the first superconducting wire forming the first superconducting conductor with the first superconducting wire, which is in a position corresponding to the first superconducting wire of the first superconducting conductor in a section of the conductor, in the superconducting wires forming the second superconducting conductor. Similarly, the respective superconducting wires forming the first superconducting conductor are joined with the superconducting wires, which are in corresponding positions, forming the second superconducting conductor.
For example, the superconducting conductors are divided into the respective superconducting wires on the end portions thereof, so that the superconducting wires to be joined are thereafter superposed with each other in constant lengths and joined with each other.
According to the present invention having the aforementioned structure, a connection structure having smaller connection resistance is obtained as compared with the case of directly connecting superconducting conductors with each other. Further, the present invention requires no sophisticated technology for the connection as compared with the case of connecting filaments with each other, dissimilarly to the prior art.
Consequently, superconducting conductors can be simply connected with each other with small connection resistance and no requirement for sophisticated technology.
Preferably, a first joined body defined by connecting the first superconducting wires forming the first and second superconducting conductors with each other is further joined with a second joined body defined by connecting the second superconducting wires forming the first and second superconducting conductors with each other.
When the joined bodies are further joined with each other, the connection between the superconducting conductors is further fixed.
Preferably, the first joined body defined by connecting the first superconducting wires forming the first and second superconducting conductors with each other may be electrically insulated from the second joined body defined by connecting the second superconducting wires forming the first and second superconducting conductors with each other.
When the joined bodies are electrically insulated from each other, the connection resistance is further reduced.
In order to electrically insulate the joined bodies from each other, a method of inserting an insulating material such as a Teflon tape between the joined bodies is employed, for example. In this case, it is preferable to employ an insulating material having a length at least identical to those of joined portions defined by two superconducting wires which are superposed with each other, and an insulating material having a length larger than those of the joined portions may be employed.
In order to electrically insulate the joined bodies from each other, a method of winding a Teflon tape or the like on the joined portion of each joined body is also employed, in place of the method of inserting the insulating material between the joined bodies. Also in this case, it is preferable to wind an insulating material having a length at least identical to those of the joined portions, and an insulating material having a length larger than those of the joined portions may be wound.
According to the present invention, the joining between the superconducting wires and the further joining between the joined bodies are attained by electrical connection with solder, for example. The solder may be prepared from Pbxe2x80x94Sn, for example.
According to the present invention, the superconducting wires can be formed by metal-coated superconducting wires, for example. A material employed for such metal coating is preferably unreactive to superconductors and easy to work, and in more concrete terms, silver or gold, or an alloy such as a gold-silver alloy, a gold-manganese alloy or a silver-antimony alloy is employed.
While the present invention is applicable to both of connection between metal superconducting conductors and that between oxide superconducting conductors, the effect of the present invention is further excellently attained in case of connecting oxide superconducting conductors with each other in particular, as compared with the prior art having remarkable disadvantages in operation.
According to the present invention, a yttrium, bismuth or thallium oxide superconductor is employed as an oxide superconductor, the bismuth oxide superconductor is most preferable in view of a high critical temperature, a high current density, low toxicity and no requirement for a rare earth element.
The present invention is not restricted to dc conductors, but is also applicable to connection of ac superconducting conductors.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.